A variable optical attenuator (VOA) is an essential component in advanced wavelength division multiplexing (WDM) telecommunication systems, which is used to adjust power variations caused by changes in source power, amplifier gain and other components. Commercially available VOA devices are mainly based on optomechanical and thermo-optic (TO) systems and usually have response times of the order of milliseconds. VOA devices based on MEMS (microelectromechanical system)[1] and TO silica[2] and polymer[3] have been reported. Both TO silica and polymers are also used in development of multi-channel planar VOA devices.
Organic and polymeric materials with desired optical properties, such as electrochromism, are deemed to be commercially useful in planar VOA devices and other integrated photonic devices. Although many electrochromic (EC) materials, including inorganic oxide (e.g., tungsten oxide), organic dye and conducting polymers (e.g., polythiophene), are known to undergo color changes in the visible region (e.g., 300-800 nm) and have potential applications in smart windows and information displays, organic materials that are electrochemically active and electrochromic in the near infrared (NIR) region or specifically within the range of the communication wavelengths (e.g., 1300-1700 nm) are less known. The application of EC materials in VOA has received very limited attention. [4]
The early work by Kaim et al. teaches that the ruthenium (Ru)-complexes of the formula (I):
with 2,2′-bipyridine (bpy) and symmetric azodicarbonyl (ADC) ligands with two identical R groups are electrochromic in the NIR region.[5] The ADC-Ru complexes prepared by Kaim have R groups which are ethoxy (OCH2CH3), benzoxy (OCH2Ph), methyl (CH3), phenyl (Ph), 4-carboxyphenyl (PhCOOH-4) and 4-methyl benzoate (PhCOOCH3-4).
When in the Ru2+/Ru3+ oxidization state, these compounds absorb strongly around 1550 nm. For example, two compounds with R=CH3 and Ph show peaks of λmax=1550 nm (ε=9330 M−1 cm−1) and 1603 nm (ε=11750 M−1 cm−1), respectively. When in the Ru2+/Ru2+and Ru3+/Ru3+ states, the complexes do not absorb in the region of 1000 and 1800 nm. The three states of these symmetric complexes can be switched from one to another by applying different potentials and bias.
Two major problems associated with these symmetric complexes (I) are (1) that the potential gap between the NIR-active state (Ru2+/Ru3+) and NIR-inactive state (Ru2+/Ru2+and Ru3+/Ru3+) is rather small, typically less than 0.57 V (or 570 mV), which makes the optical attenuation of a VOA very difficult to control electrically and (2) that these compounds do not form a thin film on an electrode (e.g., Indium-doped Tin Oxide or ITO), thus preventing the fabrication of an all-solid VOA device.
Thus, for VOA application, there is a need to have organic materials that have the chemical structures different from complexes I, but also are electroactive and are able to absorb and attenuate the light at the wavelengths of 1000 and 1800 nm.
Further, there is a need to have organic EC materials that have a large potential gap, ideally over 0.57 V or 570 mV between the NIR-active state and NIR-inactive state.
Further, there is a need to have organic EC materials that are able to form thin films on an electrode or to be deposited as thin films onto an electrode.
Finally, there is a need to have organic EC materials that can be crosslinked and form crosslinked polymeric films on electrodes for VOA device application.